1. Field of the Invention
This invention relates to an improved method and apparatus for treating a product with a liquid cryogen and more particularly to an improvement for providing efficient cooling of the cryogen vapor recovery arrangement.
2. Description of the Prior Art
The use of liquid cryogens for refrigeration and other processing of products has increased significantly with the increase in availability of cryogens, such as, for example, nitrogen, oxygen, argon, hydrogen, helium, methane, freons, carbon monoxide and carbon dioxide. One fairly recently discovered use involves the expansion of tobacco in which liquid carbon dioxide is employed as the expansion agent. A process and apparatus for so expanding tobacco are described in patent applications U.S. Ser. No. 441,767, filed by Roger Z. de la Burde and Patrick E. Aument on Feb. 12, 1974, and U.S. Ser. No. 822,793, now abandoned, filed by Larry M. Sykes and Ray G. Snow on Aug. 8, 1977, both applications being assigned to the same assignee as is the present invention. With the development of such cryogen systems, it has also become important, due in part to energy costs, to minimize the expenditure of cryogens whenever feasible. Arrangements for efficiently recovering large quantities of cryogen vapor, particularly without adversely affecting the overall treatment process have been developed. One such vapor recovery process is disclosed in U.S. Pat. No. 4,165,618 to Lewis Tyree, Jr., issued on Aug. 28, 1979.
The recovery system as described in U.S. Pat. No. 4,165,618 utilizes a plurality of gas receivers that are maintained at different predetermined pressures by means of multiple compressors. The overall system accomplishes an efficient recovery of cryogen vapor which can then be reliquified and returned to the overall cryogen treatment system. It is contemplated that the compressors in the recovery system will run substantially continuously whenever the product treatment is being carried out. One reason for such expected continuous operation is that the compressors are fairly large and starting and stopping these units are relatively timely operations affecting both production efficiency and cost.
The compressors in these recovery systems are typically connected via the gas receivers to a product processing chamber from which cryogen vapor is recovered. When the compressors are loaded, i.e., compressing the cryogen vapor withdrawn from the processing chamber, heat is removed by the flow of the cool cryogen vapor into the compressor. For continuous operation of the compressors, it is necessary in preventing overheating of the compressors that the cool cryogen vapor be withdrawn from the processing chamber fairly continuously or on a fairly regular basis without extensive delays. Thus, a problem arises when the flow of cryogen vapor to the compressors is interrupted due to unexpected equipment malfunctions, leaks or electrical breakdowns. The current recovery systems provide no compensation short of undesirably turning off the compressors to prevent frictional heat build up which could result in ineffective compressor performance or ultimately, compressor failure.
Therefore, it is desirable to provide an arrangement that will effectively cool the compressors while permitting continuous operation of the compressors during an unexpected interruption in the flow of cryogen to the compressors or during temporary maintenance periods.